Several approaches are taught in the prior art for pixel thin film transistor (TFT) circuits to deliver current to an element of a display device, such as for example an organic light-emitting diode (OLED), through a drive transistor. Conventionally, such circuits have high tolerance ranges to variations in threshold voltage and/or carrier mobility of the drive transistor by employing circuits that compensate for mismatch in the properties of the drive transistors. For example, an approach is described in U.S. Pat. No. 7,414,599 (Chung et al., issued Aug. 19, 2008), which describes a circuit in which the drive TFT is configured to be a diode-connected device during a programming period, and a data voltage is applied to the source of the drive TFT. A disadvantage of this approach is that the circuit configuration does not have high tolerance to variation in carrier mobility between different drive transistors.
Another approach is to use a known electrical current, and particularly an externally supplied “reference current”, through a drive TFT device as an element of the compensation process. For example, U.S. Pat. No. 8,284,132 (Chung, issued Oct. 9, 2012) describes applying a reference current through the drive device for compensating carrier mobility and voltage threshold variations. The circuit configuration requires two compensation phases which are time consuming, and two capacitors which need relatively large area. Such configuration, therefore, is not suitable for displays with a large number of pixels in which the compensation and programming time needs to be short.
Other approaches for compensation also have proven deficient. U.S. Pat. No. 8,643,575 (Kim, issued Feb. 4, 2014) describes applying a reference current through the drive device for compensating carrier mobility and threshold voltage variations at one capacitor, and then the data voltage is applied at another capacitor. The compensated voltage will redistribute between the two capacitors, and thus the compensation can only be partial. U.S. Pat. No. 7,812,796 (Jung, issued Oct. 12, 2010) describes applying reference current through the drive device for compensating carrier mobility and threshold voltage variations with a reference voltage at one capacitor, and then the data voltage is applied at the same capacitor. A second capacitor is used to store the compensation voltage and data voltage. This configuration requires two compensation phases which are time consuming, and two capacitors which need a relatively large area. Such configuration, therefore, is not suitable for display devices with high resolution. U.S. Pat. No. 8,405,582 (Kim, issued Mar. 26, 2013) describes applying a reference current through the drive device for compensating carrier mobility and threshold voltage variations. The voltage is measured and stored in external units. The external measurements are normally slow, which may not suitable for real time voltage threshold compensation.